Organic Field-effect Optical Waveguides

Integrating electronics and photonics on a chip is critical for high-density and high-speed optoelectronic circuits, but remains challenging. We achieve this goal by designing a new device architecture, i.e., organic field-effect optical waveguides wherein mutual modulations between electrical and optical waveguide properties are demonstrated in a device.

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For electronics, the devices using electrons as the main transmission carries can be easily controlled and tailored both temporally and spatially using electric and/or magnetic fields, which enables their ability for easily designing complex circuit architectures. The development of microelectronic technologies in 20th century has brought unprecedented great progress to mankind. Compared with electrons, photons have higher transport rate and lower energy consumption, and photonics have been regarded as the next-generation revolution for high-speed and high-capacity information and communication technology and attracted considerable attentions currently. However, the neutral and massless features of photons make it full of challenge for controlling and modulating their flows in photonic devices, which significantly limits their potential applications in integrated complex circuits. In this case, integrating electronics and photonics offers a much attractive solution for controlling the photon flows with electric field on a chip to obtain the high-density and high-speed optoelectronic circuits. But this research remains challenging due to the difficulty of merging many different areas of science and technology.

As we know, field-effect transistors wherein the field-effect is created and influenced by the gate voltage, controlling the "conducting channel" and current of the transistor are the basic component of electronic circuits and have founded today’s computer science, microelectronics and information technologies. Currently, organic field-effect transistors have received considerable attentions due to their great promising applications in organic optoelectronic devices, This is because compared with inorganic counterparts, organic semiconductors, as a new generation of semiconducting and photonic materials, take the advantages including tailoring functions by molecular design, ideal flexibility, solution processibility, low in cost as well as possessing the capacity for creating highly complex integrated optoelectronic systems on planar substrates. Based on this consideration, to achieve the goal of aforementioned integrating electronics and photonics, herein we proposed a concept of constructing organic field-effect optical waveguide (OFEW). In such device, it is expected that the third terminal-gate electrode in OFEW may realize the dual signal amplification and switching modulation on optical waveguide of organic semiconductors, vise versus.

In experiment, to probe the feasibility of our concept, OFEW device was first fabricated based on individual single crystal of a model organic semiconductor, 2,8-dichloro-5,11-dihexyl- indolo(3,2-b)carbazole (CHICZ) that simultaneously integrating efficient charge transport and superior optical waveguide ability in the same material system. Considering the requirements of measurement system with pumped lasers coming from bottom side and optical detection from top side, the whole devices were consciously constructed on ITO glass substrate (also working as the gate electrode) with transparent polyimide (PI) polymer as the insulator, which on the other hand could reduce the quenching effect of excitons at the semiconductor/insulator interface. Moreover, two ends of CHICZ single crystals in devices were both uncovered by the electrodes in order for the optical waveguide transport and signal detection. Before all the data measurement, the CHICZ single crystals were first illuminated under the measurement lasers to reach a stable sate to avoid the optical cleaning effect in device operation. Optoelectronic measurement results clearly demonstrated that the optical waveguide of CHICZ single crystals could be effectively modulated by the third terminal-the gate electrode of transistor, giving a controllable modulation depth as high as 70% and 50% in parallel and perpendicular directions of charge transport versus optical waveguide, respectively. Also, the optical waveguide with different directions can turn the field-effect of the device with the photodependence ratio up to 14800 in parallel direction. This result was also confirmed by the good repeatability in different devices. Such mutual modulation between electrical and optical waveguide properties in a device suggests the successful integration of active field-effect transistor with semiconductor waveguide modulator, which is an important step towards the goal of designing more complex optoelectronic integrated circuit architectures with desired different functionalities on a chip.

This is for the first report regarding OFEWs in organic optoelectronics, which can be extended to more complicated organic semiconductor systems. Of course, there are still many questions to solve in this field, such as the deep understanding of underlying physic mechanism in OFEWs, the basic correlation of modulation property and material structures, solid state structures as well as their intrinsic optoelectronic properties, and the control of modulation degree and directivity, especially the positive enhancement of modulation for gate electric field on optical waveguide and so on. We believe that this result will stimulate further investigations of many scientists from different fields of material science, device physics, theoretical science, organic electronics, organic photonics as well as other related research fields.

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